1. Field of the Invention
The present invention relates to a cooling structure of, in particular, a water-cooled two-cycle parallel multicylinder engine mounted on a vehicle, and to an ignition device for the same.
2. Description of the Related Art
A structure of an engine mounted on a small-sized snow vehicle such as a snowmobile, for example, is relatively simple, and a two-cycle engine which is lightweight, compact, and high-power has become a mainstream. For a two-cycle engine generally used in the snowmobile, a water-cooled engine is adopted, which prevents an overheat or an overcool and achieves an enhanced output by stably cooling the engine, and is also effective in suppressing noise.
In the two-cycle engine, as disclosed in Japanese Patent Application Laid-open No. Sho 59-180067 for example, cooling water cooled by a heat exchanger (radiator) flows through a thermostat integrated with a crankcase and is first made flow to a cylinder head by a water pump orthogonal to a crank axis. Subsequently, the cooling water flows to a cylinder and the crankcase, and then is returned to the heat exchanger.
Incidentally, in an engine of this kind, as for an ignition device in particular, an ignition plug thereof is generally disposed in a central portion of a combustion chamber. Hereby, flame is to spread evenly in the entire combustion chamber.
On the other hand, in an engine with a large cylinder bore in particular, combustion may not always be sufficiently performed with only a single ignition plug disposed in a central portion of a combustion chamber. Then, without modification, residual air-fuel mixture flows to an exhaust, causing a problem such that purification of exhaust gas becomes difficult.
Thus, conventionally, there is a means in which, for example, a plurality of ignition plugs per one cylinder are disposed to purify exhaust gas and to improve combustion. As disclosed in Japanese Patent Application Laid-open No. 2002-339745, for example, in a two-cycle engine, a plurality of ignition plugs are set in a central portion of a combustion chamber and in a position radial from the central portion.
However, in the conventional two-cycle engine or the cooling system thereof, cooling water is effective in cooling a cylinder head which becomes the highest temperature, but it is a structure such that the cooling water once warmed by the cylinder head flows to a periphery of an exhaust port which becomes the second highest temperature. Therefore, the structure is disadvantageous for a peripheral region of the exhaust port, in terms of a cooling effect.
Additionally, regarding the layout, even if the cooling water is made flow from the cylinder head, the cooling effect can be obtained in a vicinity of an upper portion of the exhaust port, but the water does not flow to a lower portion of the exhaust port, thereby forming a pouch shape. That is, it is a structure such that the cooling water is not distributed unless the water is drained to the outside of a throttle body and the like.
Further, in order to cool as far as the lower side of the exhaust port sufficiently, an amount of the water to be drained to the outside must be increased. Additionally, the drained water must be returned to the water pump in the interest of a hydraulic pressure, and as a result, warm water circulates and causes to reduce a cooling capability as the engine as a whole.
In a union structure in particular in which water is drained out of an engine at a time of engine disassembly and the like, the water cannot always be sufficiently drained due to a layout. As a result, the structure causes the water remaining in a water jacket of a crankcase runs to the inside of the engine (crankcase, in particular) when a cylinder is disengaged from the crankcase. In such a case, it becomes necessary in consequence that the entire engine is disassembled, significantly affecting maintainability.
As for the ignition device of the engine, though disposing the plurality of ignition plugs per one cylinder as described above improves a combustion efficiency and purifies exhaust gas, a relation between a flow of the cooling water of the cylinder head and disposed positions of the ignition plugs becomes a problem in a case of the water-cooled engine in particular. That is, if the plurality of ignition plugs are disposed, they may hinder the flow of the cooling water by contrast. On the other hand, in a case of a single ignition plug, a good combustion may not always be performed as described above.
Also, depending on a disposing method (position) of the plurality of ignition plugs, the ignition plugs themselves or attachment/detachment directions of plug caps may overlap among the cylinders. Thus, in the conventional ignition devices, the fact is that improving both combustion and a cooling capability or a handlability and the like is not always easy.